This invention relates to improvements in rock bolting, more particularly to a drive nut, i.e. a combination nut and removable insert adapted for use with a rock bolt.
The rock bolting art used in underground mining operations its well developed and various nuts for use with rock bolts have been previously proposed and used. Relevant prior art materials include U.S. Pat. Nos. 4,708,550 (Australian equivalent patent No. 538338), 4,295,761, 3,979,918 (Australian equivalent patent No. 487989) and Australian patent No. 539084.
Briefly, a roof of an underground mine is secured, using the rock bolting technique, by the insertion into the roof of a series of rock bolts spaced apart the appropriate distance to accommodate the particular roof material being secured. Each rock bolt is inserted by first drilling a hole in the roof to accommodate the bolt, then inserting in the hole a container of resin. The bolt is then driven into the hole to puncture the walls of the resin container and mix the resin to secure the bolt in the hole once the resin has set. A drive nut (with a displaceable insert) is used firstly at the end of the bolt where it rotates with the bolt and remains threaded to the end of the bolt until the mixed resin components set. Thereafter a further torque is applied to the nut, sufficient to rotate the nut relative to the now fixed bolt, thus ejecting the insert from the nut which is run along the bolt to firmly affix a plate washer to the mine roof.
Breaking torque of the nut with the displaceable insert (known in the art as a drive nut) is critical and the breaking torque range required for some conditions may be relatively narrow.
The main factors which determine required breaking torque of the drive nut are:
type of roof bolting equipment being used, i.e. capabilities of the motor (its torque output) PA1 amount of resin being used per rock bolt PA1 viscosity of the resin PA1 length of the rock bolt. PA1 120-150 Nm PA1 90-120 Nm PA1 65-90 Nm PA1 45-65 Nm PA1 35-45 Nm
For example, breaking torque of the drive nut has an upper limit for some equipment which must not be so high that the machine will not break the drive nut and a lower limit to prevent premature breaking of the drive nut which would result in the resin not being mixed properly and/or the bolt being pulled out of the hole by tightening of the nut prior to setting of the resin.
Typical breaking torque ranges required for standard M24 drive nuts are as follows:
Consistency of the breaking torque of the drive nut is of first importance.
The prior art exhibits many drive nuts useful in the rock bolting art but no currently Known nut and insert combination has proven entirely satisfactory. For example, a nut with a plastics sleeve and wedge insert forced into the threaded portion thereof has been used but has been found unsatisfactory in practice since control of breaking torque has been difficult. A nut with a metal insert located in the threaded part of the nut has been tried but thread damage to the nut has resulted when the metal on metal contact between nut thread and insert has been broken under torque. Further, a nut having a resin plug insert formed by pouring settable resin into the nut has been proposed and used but again, completely accurate estimation of breaking torque force has been difficult, variations arising from such difficult control variables as different bolt end shapes and plug thickness.
In particular, the nut and insert combination of U.S. Pat. No. 4,295,161 has been tested extensively and has proven to be unsatisfactory for various reasons. Breaking torque of the nut has not been consistent and greatly depends on the shape of the bolt end. The shape of the bolt end varies due to the way the round bar is sheared (prior to thread rolling) during the manufacturing process. The main factors affecting the shear mode are the sharpness and clearance of the shear blades. It is obviously difficult to maintain these two parameters as constants. A second and more important factor affecting the bolt end shape is the bar steel quality. High tensile steel bar is more brittle and will result in a quite different end shape when compared with a mild steel bar. Steel quality will also affect the shape of the bolt end during thread rolling. The end of the mild steel bolt will have a rim created by the steel being "pushed" by the thread rollers whereas a high tensile steel bolt would tend to be more "square". Thus the shape of the bolt end is practically beyond control. Typical bolt ends are depicted in FIG. 1 of the accompanying drawings. It will be noted that prior art U.S. Pat. No. 4,295,761 incorporates a bolt with a frangible thin cylindrical disk adapted to be normally retained in the cavity in the head of the nut. The variation of the breaking torque of the nut with the flat insert, resulting from variations in bolt end shape, was simply too great to allow the nut and insert combination of U.S. Pat. No. 4,295,761 to be effective and acceptable. Typically, bolt end shapes of FIGS. 1C and 1D would result in breaking torque 30-40 percent lower than that experienced with the bolt end shape depicted in FIG. 1A.
A further very significant shortcoming of the nut and flat insert combination as proposed in U.S. Pat. No. 4,295,761 resides in the fact that during extrusion of the insert from the nut, the start of the thread on the bolt is damaged with the result that a second nut cannot be screwed onto the bolt. A second nut is often threaded onto a rock bolt underground to fix some mining gear, such as ventilation equipment, pipes, cables, etc. The thread damage on the bolt occurs most noticeably when a high torque nut (90-120 Nm) with a flat insert is used in conjunction with a mild steel bolt. This arises since the flat insert when extruded from the nut forms a cap which rubs against the thread damaging the thread start. Occasionally the cap "sticks" to the end of the bolt very firmly (i.e. cannot be removed by hand), which also prevents the second nut from being screwed onto the rock bolt.
It is an object of this invention to provide an improved drive nut for use with a rock bolt. It is desirable to reduce the contact area between the insert and the bolt when the nut and insert combination is threaded onto a bolt, so that the rim on the bolt end (FIG. 1B) has no adverse affect on the breaking torque. The insert useful in the drive nut of this invention incorporates a peripheral rim and a central portion displaced relative to the plane of the rim so that first contact between a drive nut and a bolt to which the nut is threaded, is with the insert central portion. The peripheral rim of the insert is preferably annular but may be of any other suitable shape such as hexagonal, duodecagonal, etc. Further, the insert shaped in accordance with this invention is much stiffer than conventional flat inserts allowing for relatively uniform force distribution across the insert and thus shapes shown in FIGS. 1C and 1D do not affect the torque either. Clearly there can no longer, with the novel insert of this invention, be any damage to the bolt thread since there is no contact between the thread and the insert. Further, the novel insert of this invention does not "stick" to the bolt since it is never capped.
The current invention provides improvement in respect of the configuration of the nut to which this invention relates. Prior art U.S. Pat. No. 4,295,761 incorporates a protrusion in the head of the nut and the insert is placed in the recess provided by this protrusion and held therein by a crimped protrusion lip. This way of crimping of the nut protrusion has been found unsatisfactory since small variations in the crimping force will result in significant change in the breaking torque of the drive nut. The nut of this invention is forged including an integral upstanding protrusion which incorporates a recess adapted for location therein of an insert. When the insert is located in the recess, the protrusion is crimped to retain the insert in the head of the nut, that crimping being effected preferably by pressing the protrusion by a pressing tool having a number of preferably square bars. These bars press the material (steel) of the protrusion, pushing it over the insert, forming a plurality of lips to hold the insert in the nut. These "lips" are separated in the crimped protrusion by successive portions of the original protrusion which are not crimped to the same extent as the "lips". Typically, six lips of 4 mm width and 1.8 mm depth will result in 90-110 Nm breaking torque whereas the same configuration but a different depth of the lips (achieved by providing a different load in the assembling press) of 1.5 mm will lead to a breaking torque of 70-90 Nm, using a high tensile steel insert. It will be apparent to the man skilled in the art that the same torque ranges can be achieved by different combinations between the number, shape and dimensions of the lips. Torque ranges can also be adjusted by differing materials and thickness of the insert.